The present invention relates to a hand shear having one or more cutting edges of an improved wear-resistant composition. More specifically, said wear-resistant composition comprises tungsten carbide modified with selected other materials.
U.S. Pat. No. 3,451,791 and U.K. Pat. No. 1,231,084 disclose ultrafine grained cobalt-bonded tungsten carbide compositions containing 1 to 30 percent by weight of cobalt, some of which are said to be wear resistant, impact resistant, extremely hard and strong. The compositions are disclosed as useful in a wide variety of applications, many of which do not involve cutting; cutting tools are mentioned as a potential utility but no enabling disclosure is provided therefor. Compositions containing 15 to 30 percent cobalt are disclosed as having higher impact resistance and toughness but lower strength than compositions containing less than 15% cobalt. The compositions are said to be suitable for uses where tool steels are normally employed, as in dies and punches.
Conventional means for cutting high-strength fibers or fabrics prepared from said fibers are seriously deficient in wear resistance. Present hand shears are made of low carbon steels or tool steels which are heat-treated to form a hard cutting edge. The cutting edges of such tools quickly become dull and require frequent regrinding which significantly reduces the hardness and durability of the cutting edge.
The art also discloses hand shears made of zirconium oxide. These shears are very brittle and susceptible to chipping during edge grinding, and are known to shatter into fragments if dropped on a hard surface.
Another known shear consists of a mechanically held, throw-away insert attached to a holder, said insert forming the cutting edge. Misalignment common in such a device results in poor cutting.
While the above-described shears are capable of cutting many low-strength materials, they fail to provide precise blade alignment and blade edge continuity required to cut high-strength fibers such as aromatic polyamide fibers, glass and carbon and are therefore difficult and uneconomical to use for such high-strength fibers.
Some prior art cutting devices employ a relatively low carbon steel that quickly loses hardness during regrinding. Other art devices employ a hard, brittle ceramic that is difficult to grind. Moreover, the cutting edges achieved in these devices fail to furnish the sharpness and durability required to sever high-strength fibers and fabrics such as those mentioned above.
Conventional hand shears are typically manufactured using old manufacturing methods which result in high cost and poor product quality.
An object of the present invention is a cutting tool, particularly a hand shear, having a cutting edge of unusually low coefficient of friction which is superior to conventional cutting tools in edge wear resistance, hardness and/or toughness, and which is economically feasible for commercial manufacture. Additional objects include cutting tools wherein the cutting edges have chemical compositions which provide greater toughness or greater hardness, or a combination thereof, relative to prior art devices; and cutting devices which are capable of efficiently cutting high-strength fibers and fabrics prepared from said fibers.
Commonly owned U.S. Pat. 5,069,872, which is hereby incorporated by reference herein, discloses several cutting edge compositions which provide superior wear resistance and hardness. Included is the composition consisting essentially of 85% to 96% tungsten carbide and 15% to 4% cobalt. The patent discloses that tungsten carbide/cobalt compositions containing between 6% and about 13% cobalt have relatively good strength and wear-resistance. It is further disclosed that compositions containing between 13% and 25% cobalt show reduced wear-resistance, and it is concluded therein that there is no significant advantage in using tungsten carbide/cobalt compositions containing more than about 13% cobalt.